A sudden onset of non-weight-bearing lameness in a horse is always concerning, and should be evaluated as soon as possible by your veterinarian. But there is good news here. Most of these cases have a quick and positive outcome, and the horses return to soundness within a few days. That's because the majority of cases of acute lameness in one leg (with no sign of trauma or significant distress associated) are caused by a hoof abscess.
Once an abscess has burst, whether by being opened from the bottom, horses will generally improve within 24-48 hrs. While your horse will be walking better, healing still needs to happen. You should discuss further options with your vet or farrier, such as shoeing or foot bandages once the abscess is gone.
Occasionally, a diagnosed foot abscess will go on longer than a few days. In these cases, re-evaluation by a veterinarian is recommended, and radiographs may be taken. It is possible for a hoof abscess to infect the deeper structures of the hoof, including the bone. This is a serious situation, and needs immediate and aggressive treatment.
As we approach spring (and MUD season), hoof abscesses are going to become even more common. So, if you notice your normally sound horse is limping badly on one leg, don't panic. Check the bottom of the hoof for nails or other foreign objects (on a side note, if you see something actually sticking into the bottom of the foot, do your best NOT to pull it out. Protect the limb from further injury, and call a vet immediately!). Check for severe swellings which can indicate serious injury. Then feel the hoof - is it warmer than the rest? Can you feel an increased pulse at the back of the fetlock or pastern? It's probably an abscess. Call your vet for an evaluation - in a few days, your horse will probably be well on his way to recovery.